The following abbreviations are used in this disclosure:                eNB Evolved Node B        ICIC Inter-Cell Interference Coordination        LTE Long Term Evolution        NAS Non-Access Stratum        PCI Physical Cell Identity        RA Random Access        RACH Random Access Channel        RAN Radio Access Network        RAT Radio Access Technology        RRC Radio Resource Control        RSRP Reference Signal Received Power        RSRQ Reference Signal Received Quality        UE User Equipment        UTRAN UMTS Terrestrial Radio Access Network        WCDMA Wideband Code Division Multiple Access        
FIG. 1 discloses one example of a heterogeneous network 100 network suitable for use with the present invention. Improved support for heterogeneous network operations is part of the ongoing enhancements of the LTE specification of 3GPP. Heterogeneous networks may be characterized as deployments with a mixture of cells of differently sized and overlapping coverage areas. In the heterogeneous network 100, are deployed a first pico base station 122 covering a first picocell 120, a second pico base station 132 covering a second picocell 130, and a third pico base station 142 covering a third picocell 140. The picocells are deployed within a macro cell 110 of a macro base station 116 having a first antenna 112 and a second antenna 114, which may service the same or different macro cells.
A pico base station like the first pico base station 122 is a small cellular base station transmitting with low output power and typically covers a much smaller geographical area than a macro base station. A small cellular base station may be referred to as a low power node, whereas a macro base station represents a high power node. Other examples of low power nodes in heterogeneous networks are home base stations and relays.
Heterogeneous networks represent an alternative to densification of macro networks, and have classically been considered in cellular networks with traffic hotspots such as geographical areas of clustered user distributions. There, small cells covering the traffic hotspot off-load the macro cell and thus improve both capacity and the overall data throughput within the coverage area of the macro cell. In emerging mobile broadband applications, there is however a continuous demand for higher data rates and therefore it is of interest to deploy low power nodes not necessarily to cover traffic hotspots only but also at locations within the macro cell coverage where the signal-to-noise ratio prevents high data rates.
User Equipments or UEs such as, for example, cellular telephones attached to cellular networks continuously monitor which cell they should be associated with. This monitoring is typically conducted by evaluating the radio reception quality of its serving cell (current association) against radio reception quality of neighbor cells. If the radio reception quality of a neighbor cell is better than the serving cell, a new cell association is established for the user equipment.
In LTE networks, the procedures for changing cell association depend on which of the two RRC states, RRC_IDLE and RRC_CONNECTED, the UE is within. In connected mode the UE is known by the radio access network (RAN) and cell association decisions are taken by the RAN, usually based on mobility measurement reports provided by the UE. If such a report indicates that the UE is better served by a neighbor cell, then the network initiates a handover procedure.
In idle mode, the UE cannot report measurements to the network so an autonomous cell reselection procedure is used. However, the network controls the cell reselection procedure in the sense that it broadcasts cell-specific system information that UEs shall take into account when selecting a cell to camp on. Examples of system information broadcast by the network are idle mobility parameters such as thresholds (S(Non)IntraSearch in serving cell to enable measurements. q-QualMin, ThreshX,High for target cells, ThreshServing, Low for serving cell), offsets (Qoffset for carrier frequency or for individual cells) and priority lists of frequencies.
In LTE, a user may be assigned UE-specific idle mobility priorities when changing RRC state from connected to idle state. Note that priority-based cell reselection is only used between different RATs and between different carriers. One secondary objective of the priority-based selection is to minimize UE battery consumption, by reducing inter-frequency and inter-RAT measurements. Measurement quantities typically used for mobility purposes are Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ).
Depending on how these mobility measurements, possibly complemented by a configurable offset (handover bias or cell reselection offset), are used a UE may be connected/camped to the cell with the strongest received power, or the cell with the best path gain, or a combination of the two.
These different cell association principles do not always result in the same selected cell as the base station output powers of cells of different type are different. This is sometimes referred to as link imbalance. For example, the output power of a small base station or a relay is in the order of 30 dBm or less, while a macro base station may have an output power of 46 dBm. Consequently, even in the proximity of the picocell, the downlink signal strength from the macro cell may be larger than that of the picocell.
FIG. 2 discloses the macro base station 116 serving the first macro cell 110 and the third picocell 140. Typically, from a downlink perspective, it is better to select a cell based on downlink received power, whereas from an uplink perspective, it is better to select the cell based on the path loss. FIG. 2 further discloses the received or receivable power PM of the macro cell and the path loss LM of the first macro cell as a function of distance from the first macro base station 116. Likewise, the received or receivable power PP of the third picocell and the path loss LP of the third picocell as a function of distance from the third pico base station 142 are indicated in FIG. 2.
A first dotted line 202 indicates a pathloss border (or RSRP or +offset border) where the path loss LM of the first macro cell 110 is substantially equal to the path loss LP of the third picocell 140. A second dotted line 204 indicates an RSRP border where the received power PM of the first macro cell is substantially equal to the received power PP of the third picocell. The area between the pathloss border and the RSRP border is referred to as a link imbalance zone 148. Although in FIG. 2 the link imbalance zone 148 is drawn in a circular fashion, a person skilled in the art will understand that in practice, the link imbalance zone may not always have a circular shape. The reason for this is that on the right side of the third picocell 140 covered by the third pico base station 142, the pathloss border and the RSRP border may be further away from the third pico base station.
In this scenario, it may be more beneficial from the system perspective for the UE to connect to the third picocell 142 even if the macro downlink is much stronger than the picocell downlink. Increasing the coverage of small cells for operations in link imbalance zones may be done, e.g., by adding an offset or a bias to the RSRP measurements. Operations with larger offsets/biases would require Inter-Cell Interference Coordination (ICIC) across layers.
3GPP has in LTE release 10 specified new ICIC features for enabling reliable operations in link imbalance zones of UEs in the connected mode. One way of providing ICIC in 3GPP LTE release 10 is illustrated in FIG. 3.
FIG. 3 discloses a macro cell communication channel 302 and a picocell communication channel 304. Both communication channels are divided into frames, preferably with a duration of 1 millisecond.
In this case, an interfering macro cell avoids scheduling unicast traffic in certain subframes in order to create protected subframes for the picocell. The macro eNB like the macro cbase station 116 indicates via a backhaul X2 interface 150 (FIG. 1) to a neighbor pico eNB such as the third pico base station 142, which protected macro cell subframes 312 it intends to not schedule transmissions within. The pico eNB may then take this information into account when scheduling the UEs operating within the link imbalance zone; such that these UEs are prioritized to be scheduled in the protected picocell subframes 314, i.e., low interference subframes. UEs operating very close to the pico eNB may, in principle, be scheduled in all subframes.
When an idle mode UE is to connect to a cell, it is desirable that the UE stays in the connected cell for a while after transitioning to the connected state and not almost immediately trigger a handover situation. As may be observed from FIG. 2, an idle mode UE located in the link imbalance zone 148 would camp on the first macro cell 110 if the cell reselection criterion is based on the strongest received downlink signal. When such UE connects to the first macro cell, the macro base station 116 may directly hand over the UE to the third pico base station 142 if the UE is capable of operating in the link imbalance zone 148. To avoid such immediate handovers, the cell size seen by UEs capable of operating in link imbalance zones should preferably be similar in both the RRC states. One way to achieve this is to align handover biases with threshold offsets used for cell reselection.
Legacy UEs cannot be associated with picocells when they are located in link imbalance zones resulting in very low geometries. The same holds also for non-legacy UEs that do not support the new ICIC features of LTE release 10. This implies that handover biases as well as cell reselection offsets will depend on the capability of the UEs. Although a UE may be capable of being associated with a picocell when located within the link imbalance zone, it may be better from a system perspective to let such UEs camp on the macro cell if the picocell is highly loaded.
An LTE network may control traffic loads across cells in heterogeneous deployments by adjusting the density of protected resources for operations in link imbalance zones, adjusting the range/size of the link imbalance zone, and combinations of the previous two.
In the first case, the network 100 could either increase or decrease the density of protected resources in order to balance the traffic load across macro and picocells. For example, if many UEs are located within the link imbalance zone 148, the network could increase the density of protected resources in order to accommodate more UEs to the picocells and thus off-load the first macro cell.
In the second case, the network 100 could e.g. update the broadcast system information by setting new threshold offsets that either shrink or expand the range of the link imbalance zones. In situations with most UEs being located in the proximity of pico eNBs and the first macro cell being lightly loaded with respect to the picocells, the network could increase the density of protected resources and shrink the range of the link imbalance zone for idle mode UEs.
Frequent updates of broadcasted system information are not desirable so adjustments of the range of the link imbalance zone would be semi-static.